This invention relates to radio communications equipment in general and to system failure monitors in communications equipment in particular.
The increase in complexity in advanced electronic communication equipment has brought with it the need for improved maintenance techniques and methods necessary to keep an item in proper condition or to restore it to proper condition once it has failed. This includes a variety of problems particularly related to the increasing use of extremely complex solid state elements and the use of digital techniques.
The use of these advanced communications devices has particular application to aviation in that significant advances in data transfer rates, weight and cost considertions, as well as reliability, can be made through these technical advances. However, the aircraft environment under normal operating conditions provides an extremely varied temperature, pressure and mechanical stress environment. Vibration caused by the engines of the aircraft as well as repeated mechanical forces caused by landings and other normal flight-encountered air turbulence, have particularly caused problems in maintaining antenna systems on the aircraft. Repeated temperature fluctuations additionally have caused problems in antenna mounting and electrical connections.
Diversity systems have been developed primarily to reduce fading characteristics inherent in essentially all forms of electromagnetic radiation communication. Fading is a drift in the level of received radio signals beyond intelligibility. It is often caused by changes in the upper atmosphere or by increases in distance from the transmitter to the receiver; or by obstruction in the signal path, an additional reason for development of a diversity system to provide a receiving antenna which will always be illuminated by the transmitted signal. A good example of this type of diversity system is the discrete address beacom system (DABS) developed for providing the aircraft surveillance and communications necessary to support aircraft traffic control automation in the dense air traffic environments expected in the future. An excellent background of the DABS concept is provided in the Federal Aviation Administration Report No. FAA-RD-8041 published April 1980. Chapters 4 and 5 of that publication have particular application to the implementation of the present invention and are hereby incorporated by reference thereto.
The DABS transponder mounted in the aircraft, as envisaged by the above reference FAA report, incorporates the use of a diversity receiver having two antennas, one mounted on the upper side of the aircraft, and a second mounted on the underside of the aircraft. This will enable either or both of the receiving antennas to be constantly illuminated within the operating range of the ground based transmitter, irrespective of aircraft attitude relative to the horizon, speed of the aircraft, or relative position to the ground-based transmitter, notwithstanding the fading effects previously discussed.
If, however, one of the antennas fails leaving the other antenna receiver system in operation, it is desirable, for maximum serviceability of the system, to detect this problem as soon as possible. For example, if an aircraft makes a flight wherein the uppermost antenna receiver apparatus is never utilized, primarily because the lower mounted antenna apparatus remains constantly illuminated, the inoperability of the upper mounted antenna will go essentially undetected unless the aircraft on a subsequent flight assumes an attitude which will block the lower antenna from reception and, as can be clearly seen, this is the precise attitude and time of a loss of reception (such as a climb-out on take-off) when the loss in communications could be catastrophic.
A significant contributing element to the maintenance problem is the intermittent nature of certain types of failures. For example, a broken antenna connector cable may provide a low impedance contact at ground level; however, upon attaining altitude and after a period of flight wherein the aircraft is exposed to much lower ambient temperature than experienced at ground level, the broken connector becomes separated and essentially appears as a high impedance or open contact. Upon returning to ground, the warmer temperatures cause the broken connector to once again come into contact and thereby make the receiver apparatus functional once again. The first problem, that is of detection, become difficult utilizing a ground test procedure because in fact the failure only exists under circumstances attained during flight. A second problem exists in attempting to locate and determine the cause of the failure once the fact of a failure has been established. This intermittent type failure poses a significant safety hazard in a crowded air traffic environment, primarily because of the difficulty of detection of such failure.